There are two rice varieties, lowland rice and upland rice. In Japan, 97% of rice plants are lowland rice grown in paddy fields. Cultivation and management of rice plants are conducted as follows: 35 to 50 days in a lowland nursery, 30 to 40 days from transplanting to heading, and 40 to 50 days from heading to harvest. It is said that about 10.5 kg of nitrogen is required for yielding 500 kg of unmilled rice, and 6.3 kg of nitrogen is taken up from soil and water, and the remaining 4.2 kg from fertilizers. Fertilizers used are mainly ammonium sulfate-based chemical fertilizers, and about 40% of the nitrogen applied is thought to be taken up and utilized. The remaining approximately 60% of the nitrogen is left in soil, and outflow of nitrogen into groundwater due to overuse of fertilizers and the influence of residual fertilizers on rivers and lakes have become social issues.
In nature, there are microorganisms called root nodule bacteria that fix nitrogen in the air and supply the nitrogen to plants. For example, Rhizobium, Bradyrhizobium, Mesorhixobium, and Sinorhizobium nodulate and reside in the roots of legumes, and take up nitrogen gas that accounts for four-fifths of the atmospheric air. They convert nitrogen into nitrogen compounds and supply them to plants. The root nodule bacteria of soybean belong to Bradyrhizobium. 
Plants can be cultivated by delivering atmospheric nitrogen through such microorganisms, while maintaining the supply of nitrogen from the soil at a minimal level. These techniques are already being widely used, and seeds of plants, such as soybean, that are inoculated with root nodule bacteria are now commercially available. However, these techniques are not available for rice plants, since root nodule bacteria specifically infect legumes, but not poaceous plants.